High purity water is required for many industrial and utility applications including process feed water, high purity rinse water for electronic components, and demineralized water for steam turbines.
Also, many aqueous feedstocks or products must meet contamination specifications requiring ion exchange polishing to remove the undesired ionic species. The chemical process streams in amino acid production, corn (high fructose corn syrup production) and sugar refining, pharmaceutical manufacture, hydro-metallurgical production, and boiler feed water treatment are a few examples.
Typically, such purity requirements are met on an industrial scale by using large ion exchange beds, often in the range of 250 ft.sup.3. Each bed is filled with ion exchange resin. When a solution containing cation impurities like Na.sup.+, Ca.sup.2 +, Mg.sup.2 +, or Al.sup.3+ is passed through a cation exchange bed, H.sup.+ from the cation exchange resin replaces the cationic impurities present in the solution, and the impurities are left behind in the ion exchange bed. When the available H.sup.+ has been exchanged, the resin is "spent" and must be replaced or regenerated.
Similarly, in an anion exchange column, OH.sup.- is replaced by anionic impurities like Cl.sup.-, SO.sub.4.sup.2 -. When the available OH.sup.- has been exchanged, the anion exchange resin is "spent" and must be replaced or regenerated.
A cation exchange column is regenerated by passing a strong acid, usually HCl or H.sub.2 SO.sub.4 over the bed. The Cl.sup.- or SO.sub.4.sup.2 - picks up the cationic impurities from the spent resin bed, and leaves behind fresh H.sup.+. Similarly, an anion exchange column is regenerated by passing a strong base, usually NaOH, over the anion exchange beds. An excess (up to ten percent or more of the exchange capacity) of the regenerant is used in order to insure complete regeneration. In a typical water demineralization plant large quantities of acid and base are used, resulting in large quantities of salt and dilute acid or dilute alkali in the exhausted regenerant waste streams. Disposal of such waste streams, especially the anionic waste regenerant is environmentally undesirable, and the acid and base used to regenerate the exchange beds are costly. Moreover, prior art processes require storage of large quantities of acid, base or salt.
U.S. Pat. No. 4,880,513 discloses using a three compartment electrodialytic water splitter to generate HCl and NaOH for use as regenerant for ion exchange resins. Spent anion and cation regenerant may either be thrown away, or mixed and added to the constantly recirculating salt loop of the water splitting process. However, this process requires the purchase of large quantities of feed salt (NaCl) which is used to generate and maintain the circulating salt solution at a specified concentration. This process also requires pretreatment of the salt solution to remove Ca.sup.2 + and Mg.sup.2 +.
U.S. Pat. No. 4,976,838 discloses a multi chamber two compartment base purification unit and a method for using the same for the purification of a base from a stream containing free base in a salt solution. Further, U.S. application, Ser. No. 278,062 (filed Nov. 30, 1988 ) discloses a multichamber, two compartment acid purification unit and a method for using the same to separate and purify a strong acid from a feed stream containing free acid and salt. Processes for producing acid and base from the depleted salt in a three compartment water splitter are also disclosed in U.S. Pat. No. 41,740,281.
Electrodialysis units used for salt concentration or desalination are also known. U.S. Pat. No. 4,995,956 discloses a two compartment electrodialysis unit and method for using the same to regulate the concentration of the salt stream fed into an electrodialytic water splitter.
However, the previously existing processes have several drawbacks. Mixing of anion and cation regenerants prior to treatment results in further dilution of the spent regenerant solution, increased pretreatment steps and costs, and a corresponding decrease in membrane life, especially in the case of cation membranes which are particularly susceptible to fouling by insoluble cation salts. Moreover, when exhausted regenerants are combined an excess of acid is produced which must be stored or purified and concentrated for sale. The use of a constantly circulating salt solution, with a fixed concentration requires the addition of large amounts of extraneous salts, which increases the pretreatment steps, the cost of running the process, as well as the amount of feed which needs to be treated.
Furthermore, the existing processes provide little freedom for adaptation to various ion exchange process needs and, none of the above references discloses a method for substantially minimizing the total regenerants and regenerant precursors (such as solid salt) which must be purchased and stored. In short, a simplified purification process capable of ready integration into a variety of ion exchange processes was heretofore unknown.